Fix for 3GPP issue 821 Synthesis quality issues for multi channels at low bit rate [13.2 to 32 kbps]

• Fix for issue #821 (closed)

Pipeline results:

• EVS 26.444 BE test passes.

• decoder pytests:
  MLD scores:
  Original inputs: 562 failed, 374 skipped
  MLD > 10: 37
  MLD > 5 : 94
  MLD < 5 : 431
  +10dB inputs: 562 failed, 374 skipped
  MLD > 10: 149
  MLD > 5 : 185
  MLD < 5 : 228
  -10dB inputs: 562 failed, 374 skipped
  MLD > 10: 8
  MLD > 5 : 44
  MLD < 5 : 510

• encoder pytests results:
  MLD scores:
  Original inputs: 562 tests:
  MLD > 10: 15
  MLD > 5 : 47
  MLD < 5 : 500
  +10dB inputs: 562 tests:
  MLD > 10: 47
  MLD > 5 : 125
  MLD < 5 : 390
  -10dB inputs: 562 tests:
  MLD > 10: 5
  MLD > 5 : 21
  MLD < 5 : 536